Avipox or avipoxvirus is a genus of closely related pox viruses which infect fowl. The genus avipox includes the species fowlpox, canary pox, junco pox, pigeon pox, quail pox, sparrow pox, starling pox, and turkey pox. The species fowlpox infects chickens, and is not to be confused with the human disease called chickenpox. The genus avipox shares many characteristics with other pox viruses and is a member of the same subfamily, poxviruses of vertebrates, as vaccinia. Pox viruses, including vaccinia and avipox, replicate within eukaryotic host cells. These viruses are distinguished by their large size, complexity, and by the cytoplasmic site of replication. However, vaccinia and avipox are different genera and are dissimilar in their respective molecular weights, their antigenic determinants, and their host species, as reported in Intervirology Vol. 17, pages 42-44, Fourth Report of the International Committee on Taxonomy of Viruses (1982).
The avipox viruses do not productively infect non-avian vertebrates such as mammals, including humans. Further, avipox does not propagate when inoculated into mammalian (including human) cell cultures. In such mammalian cell cultures inoculated with avipox the cells will die because of a cytotoxic effect, but show no evidence of productive viral infection.
The inoculation of a non-avian vertebrate such as a mammal with live avipox results in the formation of a lesion at the inoculation site which resembles a vaccinia inoculation. However, no productive viral infection results. Nevertheless, it has now been found that a mammal so inoculated responds immunologically to the avipox virus. This is an unexpected result.
Vaccines composed of killed pathogen or purified antigenic components of such pathogens must be injected in larger quantities than live virus vaccines to produce an effective immune response. This is because live virus inoculation is a much more efficient method of vaccination. A relatively small inoculum can produce an effective immune response because the antigen of interest is amplified during replication of the virus. From a medical standpoint, live virus vaccines provide immunity that is more effective and longer lasting than does inoculation with a killed pathogen or purified antigen vaccine. Thus, vaccines composed of killed pathogen or purified antigenic components of such pathogens require production of larger quantities of vaccine material than is needed with live virus.
It is clear from the foregoing discussion that there are medical and economic advantages to the use of live virus vaccines. One such live virus vaccine comprises vaccinia virus. This virus is known in the prior art to be a useful one in which to insert DNA representing the genetic sequences of antigens of mammalian pathogens by recombinant DNA methods.
Thus, methods have been developed in the prior art that permit the creation of recombinant vaccinia viruses by the insertion of DNA from any source (e.g. viral, prokaryotic, eukaryotic, synthetic) into a nonessential region of the vaccinia genome, including DNA sequences coding for the antigenic determinants of a pathogenic organism. Certain recombinant vaccinia viruses created by these methods have been used to induce specific immunity in mammals to a variety of mammalian pathogens, all as described in U.S. Pat. No. 4,603,112, incorporated herein by reference.
Unmodified vaccinia virus has a long history of relatively safe and effective use for inoculation against smallpox. However, before the eradication of smallpox, when unmodified vaccinia was widely administered, there was a modest but real risk of complications in the form of generalized vaccinia infection, especially by those suffering from eczema or immunosuppression. Another rare but possible complication that can result from vaccinia inoculation is post vaccination encephalitis. Most of these reactions resulted from inoculating individuals with skin diseases such as eczema or with impaired immune systems, or individuals in households with others who had eczema or impaired immunological responses. Vaccinia is a live virus, and is normally harmless to a healthy individual. However, it can be transmitted between individuals for several weeks after inoculation. If an individual with an impairment of the normal immune response is infected either by inoculation or by contagious transmission from a recently inoculated individual, the consequences can be serious.
Thus, it can be appreciated that a method which confers on the art the advantages of live virus inoculation but which reduces or eliminates the previously discussed problems would be a highly desirable advance over the current state of technology. This is even more important today with the advent of the disease known as acquired immune deficiency syndrome (AIDS). Victims of this disease suffer from severe immunological dysfunction and could easily be harmed by an otherwise safe live virus preparation if they came in contact with such virus either directly or via contact with a person recently immunized with a vaccine comprising such a live virus.